Efficient semiconductor manufacturing requires highly precise and accurate metrology instruments. In particular, a metrology instrument is required to achieve small tolerances to achieve better quality products and fewer rejections in the manufacturing process. For example, the 1999 Edition of the International Technology Roadmap for Semiconductors lists the necessary precision needed for isolated line control in the year 2001 to be 1.8 nm. Unfortunately, correctly assessing and optimizing the measurement potential of a metrology instrument is difficult for a number of reasons. For example, an evaluator normally has limited access to the various instruments under consideration. In addition, each instrument needs to be evaluated under a wide range of conditions in order to gain a valid impression of how it will perform in the actual manufacturing setting. Finally, there are no widely accepted standards relative to the required parameters and how the parameters should be measured. As a result, an adequate solution for calculating an uncertainty of a metrology instrument in meaningful units of length for comparison to manufacturing lithography requirements has been elusive.
Current assessment methods are often based on the repeatability and reproducibility (R&R) of an instrument. For a critical dimension (CD) metrology instrument, evaluation is often executed by pulling representative samples of partially constructed product wafers from a manufacturing line. Recipes (programming instructions) are then implemented on an instrument under evaluation such that estimates of the static repeatability and long term reproducibility can be made. For example, to determine static repeatability for a measurement of a given product level, a recipe is implemented to cause the CD metrology instrument to navigate to a particular site on the wafer and then repeatedly measure the size of a given feature. The measurement repeatability is determined from the standard deviation of the acquired data. Long term reproducibility, also called precision, is determined in a similar way to static repeatability except that between each measurement the sample is removed from the instrument for an arbitrary length of time ranging from seconds to days. Unfortunately, the repeatability and reproducibility of a measurement is meaningless if the measurement is wrong. Accuracy must also be considered. The above-described methods do not evaluate the accuracy of an instrument apart from ensuring proper magnification by calibration with pitch standards. The reason, in part, that accuracy is not considered is that accepted accuracy standards are generally not available because the speed at which semiconductor technology advances usually makes any standard obsolete very quickly. The result of these methodologies is that a measurement system under test may be misleadingly denoted as trustworthy.
One proposed solution for metrology instrument assessment introduces new parameters related to accuracy in addition to precision. See Banke and Archie, “Characteristics of Accuracy for CD Metrology,” Proceedings of SPIE, Volume 3677, pp. 291-308 (1999). This approach deviates from using standard product wafers as samples by, for example, constructing wafers referred to as focus and exposure matrix (FEM) wafers. In this methodology, the actual CD value is determined for various fields on the FEM by using a respected reference measurement system (RMS). Following this approach, the RMS values and measurements from the instrument under test are compared by a linear regression method that is valid for situations where both variables are subject to error. Use of the FEM wafers is advantageous because they provide examples of product variation that under normal manufacturing line circumstances may occur only after a considerable time has passed. Important parameters of this methodology include the regression slope, the average offset, and a “poorness-of-fit” parameter called nonlinearity. Despite the existence of this suite of parameters for repeatability, reproducibility and accuracy, however, an evaluator must still determine, somewhat arbitrarily, how to combine these various parameters to assess or optimize an instrument.
In view of the foregoing, there is a need in the art for improved methods of assessing and optimizing metrology instruments.